The Impacts of Erroneous Freezing Fog Observations on Aircraft Ground Icing Operations

In recent years, mixed phase precipitation events at major airports have contributed to a significant number of delayed and canceled flights. Many of these have been due to a lack of aircraft anti-icing fluid holdover times in the Federal Aviation Administration (FAA) and Transport Canada (TC) Holdover Time (HOT) tables in varying types of mixed-phased precipitation conditions. The National Center for Atmospheric Research (NCAR) ground icing research team analyzed 20 years of METAR precipitation type observations from approximately 150 ASOS stations where human observers were available to augment the weather observations in the United States. Although freezing fog (FZFG) is considered an obscuration in the METARs and not a precipitation type, it was still included in the analysis because an aircraft taxiing on the ground through freezing fog conditions can accrete ice on critical aircraft surfaces, requiring the aircraft to undergo deicing and anti-icing processes.

In conducting this mixed precipitation type evaluation, several issues with present weather type reporting in the U.S. became apparent. For example, moderate intensity snow (SN) mixed with FZFG was one of the top five most reported mixed conditions in the United States. Further analysis indicated that the requirements for the Automated Surface Observing System (ASOS) and the Automated Weather Observing System (AWOS) to report FZFG were also met when the systems reported moderate and heavy snow. This meant that FZFG can be reported during moderate and heavy SN conditions, even if FZFG isn’t present. This is significant because recently published HOT tables indicate that anti-icing fluid protection times are reduced by approximately half under SN FZFG conditions, as opposed to just SN conditions, and present a considerable impact to ground aircraft operations. This presentation will focus on the automated systems methods for reporting FZFG and SN, highlighting the inaccurate reporting of FZFG during moderate and heavy snow conditions and its prevalence, as well as discussing the aviation impacts of reporting FZFG with SN when FZFG isn’t occurring.

This research is in response to requirements and funding by the FAA. The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.